Variable capacitors may be useful, but the air gap that provides their capacitance is their greatest weakness. Rather than deal with the poor dielectric properties of air, some high-end variable capacitors replace it with a vacuum, which presents some obvious mechanical difficulties, but does give the resulting capacitor a remarkable quality factor, high-voltage performance, and higher capacitance for plate area than their air-gapped brethren. [Shahriar] of [The Signal Path] managed to acquire a pair of these and took a detailed look at their construction and performance in a recent video.
For a hacker, power is pretty fundamental, so it behooves us to know a little bit about what our options are when it comes time to regulate power to our projects. In this video [Alex Hanson] from Texas Instruments runs us through the linear voltage regulators known as low-dropout regulators (LDOs). It turns out that LDOs are often a poor choice for voltage regulation because they are inefficient when compared to switching regulator alternatives and can be more expensive too.
So when might you use an LDO? In very low power situations where heat and efficiency doesn’t matter very much. LDOs operate best when the input voltage is very near the output voltage and when current demands are low (roughly speaking less than ~50 mA is okay, ~500 mA is maximum, and some applications will support 1 to 3 A, although not with great efficiency and in this case thermal emissions — or magic smoke! — will become an issue).
What LDOs bring to the table is relatively clean and low-noise voltage as well as low dropout voltage (the minimum difference between the input and output voltage needed for regulation), which is their defining feature. What’s more with an appropriate output capacitor they can react quickly to load changes and they usually emit minimal EMI. LDOs are not about efficiency, they are about quality, simplicity, and control.
If you’ve seen a big air-variable capacitor, you may have noticed that some of the plates may have slots cut into them. Why? [Mr Carlson] has the answer in the video below. The short answer: you can bend the tabs formed by the slots to increase or decrease the capacitance by tiny amounts for the purpose of tuning.
For example, if you have a radio receiver with a dial, you can adjust the capacitor to make certain spots on the dial have an exact frequency. Obviously, you can only adjust in bands depending on how many slots are in the capacitor. Sometimes the adjustments aren’t setting the oscillator’s frequency. For example, the Delco radio he shows uses the capacitor to peak the tuning at the specified frequency.
You usually only find the slots on the end plates and, as you can see in the video, not all capacitors have the slots. Of course, bending the plates with or without slots will make things change. Just don’t bend enough to short to an adjacent plate or the fixed plates when the capacitor meshes.
Everyone loves a full-wave bridge rectifier, but there’s no denying that they aren’t 100% efficient due to the diode voltage drop. Which isn’t to say that with some effort we cannot create an ideal bridge rectifier using active components, as demonstrated by [Mousa] with an active bridge circuit. This uses the NXP TEA2208T active bridge rectifier controller, along with the requisite four MOSFETs.
Comparing a diode bridge rectifier with an active bridge rectifier. (Credit: Mousa, YouTube)
Taking the circuit from the datasheet, a PCB was created featuring four FDD8N50NZ MOSFETs in addition to the controller IC. These were then compared to a diode-based bridge rectifier, showing the imperfections with the latter when analyzing the output using an oscilloscope.
As expected, the active rectifier’s output was also one volt higher than the diode bridge rectifier, which is another small boost to overall efficiency. According to NXP’s product page, there’s about a 1.4% efficiency gain at 90 VAC, with the chip being promoted for high-efficiency operations. When you consider that many designs like computer PSUs feature one or more diode bridge rectifiers often strapped to heatsinks, the appeal becomes apparent. As for [Mousa], he put this particular board in his laboratory PSU instead of the diode bridge rectifier, because why not.
Perhaps the biggest impediment to using an active rectifier is the cost, with the TEA2208T coming in at $4 on DigiKey for a quantity of 100, in addition to the MOSFETs, PCB, etc. If power efficiency isn’t the goal, then some wasted power and an aluminium heatsink is definitely cheaper.
LED bezels (also known as LED panel-mount holders) are great, so how about 3D printing the next ones you need? Sure, they’re inexpensive to purchase and not exactly uncommon. But we all know that when working on a project, one doesn’t always have everything one might need right at hand. At times like that, 3D printing is like a superpower.
Printing a part you find yourself short of can be a lifesaver.
[firstgizmo]’s design is made with 3D printing in mind, and most printers should be able to handle making them. Need something a little different? You’re in luck because the STEP files are provided (something we love to see), which means modifications are just a matter of opening them in your favorite CAD program.
There’s not even any need to export to an STL after making tweaks, because STEP support in slicer programs is now quite common, ever since PrusaSlicer opened that door a few years ago.
Not using 5 mm LEDs, and need some other size? No problem, [firstgizmo] also has 3 mm, 8 mm, and 10 mm versions so that it’s easy to mount those LEDs on a panel. Combined with a tool that turns SVG files into multi-color 3D models, one can even make some panels complete with color and lettering to go with those LEDs. That might be just what’s needed to bring that midnight project to the next level.
If you read about Hall effect sensors — the usual way to detect and measure magnetic fields these days — it sounds deceptively simple. There’s a metal plate with current flowing across it in one direction, and sensors at right angles to the current flow. Can it really be that simple? According to a recent article in Elektor, [Burkhard Kainka] says yes.
The circuit uses a dual op amp with very high gain, which is necessary because the Hall voltage with 1 A through a 35 micron copper layer (the thickness on 1 oz copper boards) is on the order of 1.5 microvolts per Tesla. Of course, when dealing with tiny voltages like that, noise can be a problem, and you’ll need to zero the amplifier circuit before each use.
The metal surface? A piece of blank PCB. Copper isn’t the best material for a Hall sensor, but it is readily available, and it does work. Of course, moving the magnet can cause changes, and the whole thing is temperature sensitive. You wouldn’t want to use this setup for a precision measurement. But for an experimental look at the Hall effect, it is a great project.
Today, these sensors usually come in a package. If you want to know more about the Hall effect, including who Edwin Hall was, we can help with that, too.
Currently quartz crystal-based oscillators are among the most common type of clock source in electronics, providing a reasonably accurate source in a cheap and small package. Unfortunately for high accuracy applications, atomic clocks aren’t quite compact enough to fit into the typical quartz-based temperature-compensated crystal oscillators (TCXOs) and even quartz-based solutions are rather large. The focus therefore has been on developing doped silicon MEMS solutions that can provide a similar low-drift solution as the best compensated quartz crystal oscillators, with the IEEE Spectrum magazine recently covering one such solution.
Part of the DARPA H6 program, [Everestus Ezike] et al. developed a solution that was stable to ±25 parts per billion (ppb) over the course of eight hours. This can be contrasted with a commercially available TCXO like the Microchip MX-503, which boasts a frequency stability of ±30 ppb.
Higher accuracy is achievable by swapping the TCXO for an oven-controlled crystal oscillator (OCXO), with the internal temperature of the oscillator not compensated for, but rather controlled with an active heater. There are many existing OCXOs that offer down to sub-1 ppb stability, albeit in quite a big package, such as the OX-171 with a sizable 28×38 mm footprint.
With a MEMS silicon-based oscillator in OXCO configuration [Yutao Xu] et al. were able to achieve a frequency stability of ±14 ppb, which puts it pretty close to the better quartz-based oscillators, yet within a fraction of the space. As these devices mature, we may see them eventually compete with even the traditional OCXO offerings, though the hyperbolic premise of the IEEE Spectrum article of them competing with atomic clocks should be taken with at least a few kilograms of salt.
